This invention relates to estimating the amount of blood flowing to the tissues of the kidney. More particularly, the invention is related to analyzing obtained power Doppler ultrasound images taken of the kidney and even more specifically of the kidney cortex tissue to produce a numerical value that correlates to the blood flow.
Patients suffering from shock have impaired blood flow to the vital organs. The kidney is one such organ. A goal of therapy is to restore blood flow to these organs, including the kidney and in particular the cortex thereof.
Pulmonary artery catheters are often placed during complicated resuscitations, but they carry finite risks such as pneumothorax, pulmonary artery laceration, pulmonary infarction, and line sepsis. Also, data derived from pulmonary artery catheters, arterial blood gases, and so forth address global perfusion but not regional organ perfusion, whereas the latter may be more important in resuscitation.
Shock may result from a burn. Most thermally injured patients respond to standard resuscitation regimens, in which a physiologic crystalloid solution is infused at a rate dictated by total burn surface area and weight, with titration of that rate based primarily on the adequacy of the urine output. However, this approach fails on occasion, particularly in massively burned patients. A recent review at the U.S. Army Institute of Surgical Research Burn Center revealed that 12 of 93 nonsurviving burn patients (13 percent) were resuscitation failures, in whom hemodynamic stability could not be achieved as discussed by Cioffi et al. in xe2x80x9cCause Of Mortality in Thermally Injured Patients,xe2x80x9d Die Infektion beim Brandverletzten: Proceedings of the xe2x80x9cInfektionsprophylaxe und Infektionshekampfung beim Brandverletztenxe2x80x9d International Symposium, ed. S. Lorenz and P.-R. Zellner, Darmstadt: Steinkopff Verlag, 1993, pp. 7-11. To salvage these high risk patients, new monitoring devices that accurately assess tissue perfusion may be necessary.
Furthermore, in patients with acute renal failurexe2x80x94whether oliguric or non-oliguricxe2x80x94urine output does not necessarily reflect renal perfusion. This is likewise true of patients who have received a diuretic, whose urine output is driven by glycosuria or nitrogen metabolites, or those in whom alcohol has inhibited antidiuretic hormone. Also, an intervention intended to improve renal blood flow, such as the institution of an inotropic agent or a bolus of an intravenous crystalloid solution, may affect urine output in delayed fashion. Clinicians frequently estimate kidney blood flow by measuring the amount of urine produced per hour. However, in several classes of patientsxe2x80x94to include those with acute renal failure and those who have received drugs, which artificially increase the production of urinexe2x80x94urine output measurements are not reliable indicators of kidney blood flow.
No device or technique exists rapidly and reliably to measure kidney blood flow at the level of the small blood vessels of the renal cortex in humans.
One noninvasive tool that has been tried is color Doppler ultrasound (CDUS), which displays mean velocity data and is useful in the study of large vessel blood flow. The slowest flow velocity detectable using color Doppler ultrasound is approximately 4 cm/sec. The ability to detect blood flow in smaller vessels did not exist until the advent of power Doppler ultrasound (PDUS), which provides spectral analysis of the received sound and integration of the resulting amplitude-frequency function to permit display of a perfusion index for each pixel as discussed by Rubin et al. in an article entitled xe2x80x9cPower Doppler US: A Potentially Useful Alternative to Mean Frequency-based Color Doppler US,xe2x80x9d Radiology, 190:853-6 (1994) and Bude et al. in an article entitled xe2x80x9cPower Versus Conventional Color Doppler Sonography: Comparison In The Depiction Of Normal Intrarenal Vasculature,xe2x80x9d Radiology, 192:777-80 (1994). PDUS is likely to be able to detect blood flow velocity of less than 1 cm/sec in ultrasound images.
In the PDUS mode, the ultrasound processor performs spectral analysis of the reflected sound, e.g., via fast Fourier transform. The amplitude (or power) of each received frequency is proportional to the number of red blood cells (RBCs) which are reflecting at that frequency. Frequency, in turn, is proportional to the velocity of the RBCs. Thus, a large number of RBCs moving at a low velocity should generate a high-power, low-frequency signal, whereas a smaller number of RBCs moving at high velocity should generate a low-power, high-frequency signal. The processor then integrates the power of the received signal over frequency to obtain a perfusion index for each pixel as discussed by Dymling et al. in an article entitled xe2x80x9cMeasurement of Blood Perfusion in Tissue Using Doppler Ultrasound,xe2x80x9d Ultrasound in Medicine and Biology, 1991:433-44 (1991).
In vitro phantom studies have shown that PDUS image intensity depends, as expected, on both the velocity and the concentration of reflecting particles. Commercially available ultrasound devices translate this numerical data into color output, in which the magnitude of the perfusion index is represented by color intensity. This color information is superimposed on the gray scale ultrasound image as illustrated by Parro et al. in an article entitled xe2x80x9cAmplitude Information from Doppler Color Flow Mapping Systems: A Preliminary Study of the Power Mode,xe2x80x9d J Am Coll Cardiol, 18:997-1003 (1991).
Another advantage of PDUS is that it is not subject to aliasing (a signal wrap-around phenomenon seen in CDUS). The relative angle-independence of PDUS makes it less sensitive to inaccurate flow information based on an improper angle of insonation. One disadvantage of PDUS is a longer scanning time, which makes it more susceptible to motion and flash artifacts.
Clinical and animal studies employing PDUS have, as in a study conducted by the inventors and discussed later, demonstrated its utility in imaging the kidneys. Bude et al. demonstrated the increased sensitivity of PDUS over CDUS in depicting intrarenal and renal cortical flow in normal human kidneys, describing the latter as a non-pulsatile xe2x80x9cblush.xe2x80x9d Durick et al. in an article entitled xe2x80x9cRenal Perfusion: Pharmacologic Changes Depicted with Power Doppler US in an Animal Model,xe2x80x9d Radiology, 197:615-7 (1995), used an image analysis procedure to quantify changes in total renal perfusion as depicted by PDUS, following infusion of epinephrine and then papaverine into the renal artery of swine. Taylor et al., as discussed in an article entitled xe2x80x9cRenal Cortical Ischemia in Rabbits Revealed by Contrast-Enhanced Power Doppler Sonography,xe2x80x9d American Journal of Roentgenology, 170:417-22 (1998), used contrast-enhanced PDUS to measure renal cortical perfusion during hemorrhagic hypotension in rabbits. The Taylor 5 et al. study found good correlation with blood flow as measured by radiolabelled microspheres; in contrast to the study conducted by the inventors, this correlation was not found when ultrasonographic contrast injection was not performed.
Akiyama et al. In their article entitled xe2x80x9cHemodynamic Study of Renal Transplant Chronic Rejection Using Power Doppler Sonography,xe2x80x9d Transplant Proc, 28:1458-60 (1996) proposed the use of power Doppler ultrasonography to represent blood flow; which however, offered no evidence that a power Doppler image would represent microvascular blood flow. They discussed the use of a power Doppler image focused on three areas of the kidney (the interlobar artery, the interlobular artery, and a portion of the outer cortex) to compare well functioning kidneys (Sxe2x80x94Crxe2x89xa62.0 mg/dL) and poorly functioning kidneys (Sxe2x80x94Cr greater than 2.0 mg/dL) after a kidney transplant operation. The problem with this analysis, in part, is that going into the study it was known which kidneys were accepted and which kidneys were rejected by transplant patients. An explanation for the selection of these three locations is that Akiyama et al. assumed that these three areas would be more indicative of the status of the kidney than other areas of the kidney. These three sites as a result are not sufficiently indicative of the overall blood perfusion through the entire kidney. Another problem with their technique is that 32 gray-levels were utilized, thus offering less capability to accurately quantify perfusion in the PDUS image. Additionally, it is unclear from their article as to what a pixel index is and what method was used to calculate their mean number.
Furthermore, current methods of image analysis are labor-intensive and thus prone to mistakes and guess work at times.
Notwithstanding the usefulness of the above-described methods, a need still exists for a quantitative assessment of the ability of PDUS to measure changes in organ perfusion at the capillary level especially during ischemia, reperfusion, and more particularly following burn injury.
This invention provides a reliable indicator of the blood flow through the tissues of the cortex of the kidney by analyzing a PDUS image. More specifically, the intensity of the PDUS image correlates with renal cortex blood flow. This correlation exists during ischemia, reperfusion, and following burn injury.
An objective of the invention is to provide an accurate and reliable indication of the blood flow through an organ such as a kidney. A more particular objective is to predict the microvascular blood flow through the kidney.
A further objective of the invention is to have a noninvasive technique for measuring blood flow through an organ.
Another objective of the invention is to provide a quick numerical value indicating the state of microvascular blood flow through a particular organ to allow for a change in treatment of the patient, if necessary.
Another objective of the invention is to provide a numerical representation of blood flow through a kidney that may be used in conjunction with the diagnosis to determine whether there is sufficient renal cortical flow.
An advantage of the invention is the ability to determine a numerical representation of blood flow through any selected organ and particularly the kidney.
Another advantage of the invention is the determination of a numerical representation that correlates to blood flow through the entire organ by focusing on the smallest blood vessels.
Another advantage of the invention is an accuracy of a representation of blood flow that has not occurred in prior methods, in part, because of a lack of resolution.
A further advantage of the invention is the elimination of the anatomical features of the organ.